Bugfixes and security vulnerabilities, mostly. So long as fallible humans make fallible hardware running fallible software that in turn executes and/or compiles fallible code, there will always be a need for continued development of critical tooling and packages.

On a long enough timeline, those fixes become fewer and less frequent as the codebase improves, but there is no "done" in software unfortunately. Hell, entropy itself means nothing is ever done, just in an ever-changing state.